Motion control systems typically require position feedback. For example, an active magnetic bearing control may require non-contact position sensors for feedback signal in order to be able to keep the position of a rotor within allowable tolerances. Position feedback can be achieved by eddy-current, inductive, capacitive or optical sensors, for example.
Capacitive displacement sensors are frequently used in industry in a wide variety of applications including precise positioning, motion control, semi-conductor processing, and assembly of equipment, thickness measurements, tools metrology and assembly line testing.
In order to measure the distance between two conductive materials, a capacitive sensing system may utilise an equation for approximating capacitance C of a parallel-plate capacitor:C=∈0∈rS/r,  (1)where ∈0 is the permittivity of free space (electric constant), ∈r is the relative static permittivity (dielectric constant) of the insulating material in the gap between the capacitor plates, S is the area of the plates, and r is the distance between the plates.
Assuming that the area S and dielectric ∈0∈r of the material in the gap (usually air) remain constant, the capacitance value C reflects the changes in the distance r between the probe electrode and the target. The capacitive sensing system may be configured to measure the capacitance (or its changes) and therefore a distance (or motion). The current i in the capacitor is proportional to the capacitance value C and to the rate of change of voltage dv/dt across the capacitor (i=Cdv/dt).
With capacitive displacement sensors, high-resolution measurements of the position and motion of a conductive target may be achieved. In the case of position measurement of a rotor of an electrical machine, a sensor tip may be mounted around the shaft for each measured dimension (degree of freedom).
However, precise position and motion sensors may also have major disadvantages in industrial applications. The sensors may be expensive and complex to manufacture. They may be prone to mounting and assembly errors. The sensors may introduce significant delays to the measurements. Further, the sensors may have significant temperature dependence and they may be sensitive to noise. When interfacing indirectly with a motion controller or a rotor, for example, the sensor(s) may be susceptible to electromagnetic interferences and/or mechanical damage. Electromechanical runout of the rotor may also pose a major challenge for the control system.